1. Field of the Invention
The present invention relates to a multiplexed recursive filter bank.
2. Description of the Prior Art
In radar systems, where weak narrow band doppler signals are masked by wide band doppler noise, the detectability of the narrow band signal is improved by matched filters; that is, a filter where the bandwidth is chosen to produce a maximum signal to noise ratio at the output. Also, filter banks operating in the frequency range below several KHz have application in target identification. Radar target signature characteristics which permit identification based on frequency discrimination are the doppler frequency shift and spurious frequencies caused by target movement other than forward motion. These spurious frequencies tend to be unique to a particular target and can be interpreted by means of a filter bank.
Therefore, parallel filtering by a contiguous bank of matched filters is frequently used to achieve reliable detection of signals whose doppler frequency and time of occurrence are unknown. In general, the signals at the higher doppler frequencies are of shorter duration than those at lower doppler frequencies, so the matched filter bandwidth increases with frequency.
Heretofore, matched filters were used that were completely analog in nature; or were completely digital in nature; that is to say, both the weighting of the sampled pulses and the delay line, was accomplished solely, either by digital devices or analog devices. A recursive type matched filter with a single channeled output is proposed in U.S. Pat. No. 3,622,916, issued Nov. 23, 1971, which suggests utilizing addition circuits that provide for adding the sampled signal to the filter in digital form, so that the delay circuits may consist of a digital shift register. Also, U.S. Pat. No. 3,740,591 shows a charge transfer device recursive filter where the analog signal delay has been implemented by a bucket brigade device to achieve a single band pass filter. The characteristics are determined by feedback factors and the analog signal delay time. The usable pass band of such a recursive filter involves only frequencies less than the Nyquist limit, associated with the minimum sampling rate. Often the input signal of a recursive filter is over sampled by a predetermined factor, requiring that the charge transfer device analog delay line has the capability of storing and shifting the predetermined analog signal samples at a sampling rate that is equal to the predetermined over sampled factor divided by the analog signal delay time. The filter described in U.S. Pat. No. 3,740,591 does not utilize over sampling. Consequently only one filter characteristic resulted from the two pole recursive filter network.
While it is desirable to provide a single device that would provide more than one filter characteristic without the necessity of changing the constants, such as by switching resistors in and out would result in a cumbersome device. Also, it is desirable to provide a multiplexed filter that weights the analog signal samples with a digital constant without the necessity of being preceded by an analog to digital converter. Also, it is desirable to provide a filter which allows both uniform and non-uniform filter bank designs and where the filter complexity does not increase rapidly with the number of independent channels; and also where the center frequency and bandwidth of the filter are independent of the number of channels. Finally, to take advantage of the great storage capacity of charge coupled devices, the constants may be stored in a digital memory, while the weighting is accomplished in an A/D multiplier using a digital dddress to the memory for sequencing the constants.